Final answer:
Carbon enters the food web via photosynthesis, which is the process of carbon fixation that starts the carbon cycle within ecosystems.
Step-by-step explanation:
Carbon enters a food web via photosynthesis through the process of carbon fixation. This is part of the larger carbon cycle, which is one of the several biogeochemical cycles that involve the movement of essential nutrients through the living (biotic) and non-living (abiotic) components of ecosystems. In photosynthesis, plants, algae, and some bacteria use sunlight to convert carbon dioxide from the atmosphere into organic matter that becomes part of the food web. This is how carbon is first incorporated into the biosphere before it continues circulating through other processes such as respiration, decomposition, and combustion.